pt-online-schema-change: metadata lock can break database for rebuild_constraints

create table t_prnt(prnt_id int auto_increment primary key);
create table t_chld(chld_id int auto_increment primary key, prnt_id int);
alter table t_chld add constraint FOREIGN KEY (prnt_id) REFERENCES t_prnt(prnt_id) ON DELETE CASCADE;
replace into t_prnt values (1);
insert into t_chld (prnt_id) values (1);

# create an mdl against child table:
mysql -e 'begin; select sleep(3600) from test.t_chld;'

PTDEBUG=1 ./pt-online-schema-change --execute --alter-foreign-keys-method rebuild_constraints D=test,t=t_prnt --alter 'add column c int

2019-07-20T11:01:33 Swapping tables...
# Retry:3823 6891 Try 1 of 10
# pt_online_schema_change:10617 6891 RENAME TABLE `test`.`t_prnt` TO `test`.`_t_prnt_old`, `test`.`_t_prnt_new` TO `test`.`t_prnt`
....
# Retry:3835 6891 Calling wait code
# Retry:3823 6891 Try 10 of 10
# pt_online_schema_change:10915 6891 ALTER TABLE `test`.`t_chld` DROP FOREIGN KEY `t_chld_ibfk_1`, ADD CONSTRAINT `_t_chld_ibfk_1` FOREIGN KEY (`prnt_id`) REFERENCES `test`.`t_prnt` (`prnt_id`) ON DELETE CASCADE
# Retry:3829 6891 Try code failed: DBD::mysql::db do failed: Lock wait timeout exceeded; try restarting transaction [for Statement "ALTER TABLE `test`.`t_chld` DROP FOREIGN KEY `t_chld_ibfk_1`, ADD CONSTRAINT `_t_chld_ibfk_1` FOREIGN KEY (`prnt_id`) REFERENCES `test`.`t_prnt` (`prnt_id`) ON DELETE CASCADE"] at ./pt-online-schema-change line 10916.
# 
# Retry:3845 6891 Try code did not succeed
# CleanupTask:6584 6891 Calling cleanup task CODE(0x5649d741e2f8)
# pt_online_schema_change:9396 6891 Clean up triggers
2019-07-20T11:11:48 Dropping triggers...
# Retry:3823 6891 Try 1 of 10
# pt_online_schema_change:11296 6891 DROP TRIGGER IF EXISTS `test`.`pt_osc_test_t_prnt_del`
# Retry:3840 6891 Try code succeeded
# Retry:3823 6891 Try 1 of 10
# pt_online_schema_change:11296 6891 DROP TRIGGER IF EXISTS `test`.`pt_osc_test_t_prnt_upd`
# Retry:3840 6891 Try code succeeded
# Retry:3823 6891 Try 1 of 10
# pt_online_schema_change:11296 6891 DROP TRIGGER IF EXISTS `test`.`pt_osc_test_t_prnt_ins`
# Retry:3840 6891 Try code succeeded
2019-07-20T11:11:49 Dropped triggers OK.
# pt_online_schema_change:9159 6891 Clean up new table
# TableParser:3445 6891 Checking `test`.`_t_prnt_new`
# TableParser:3451 6891 SHOW TABLES FROM `test` LIKE '\_t\_prnt\_new'
# TableParser:3462 6891 Table does not exist
# pt_online_schema_change:9165 6891 New table exists: no
# pt_online_schema_change:9036 6891 Clean up done, report if orig table was altered
Altered `test`.`t_prnt` but there were errors or warnings.
Error updating foreign key constraints: 2019-07-20T11:11:48 DBD::mysql::db do failed: Lock wait timeout exceeded; try restarting transaction [for Statement "ALTER TABLE `test`.`t_chld` DROP FOREIGN KEY `t_chld_ibfk_1`, ADD CONSTRAINT `_t_chld_ibfk_1` FOREIGN KEY (`prnt_id`) REFERENCES `test`.`t_prnt` (`prnt_id`) ON DELETE CASCADE"] at ./pt-online-schema-change line 10916.

# NibbleIterator:6175 6891 Finish nibble_sth
# NibbleIterator:6175 6891 Finish explain_nibble_sth
# Cxn:4096 6891 Destroying cxn
# Cxn:4106 6891 DBI::db=HASH(0x5649d74176b0) Disconnecting dbh on 2228f53cb7c8 
# Cxn:4096 6891 Destroying cxn
# Cxn:4106 6891 DBI::db=HASH(0x5649d7410848) Disconnecting dbh on 2228f53cb7c8